Changing the face of Charleston : The unbuilding of the Grace and Pearman Bridges
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July 14, 2006: C-1 base makes its way to the bottom

About underwater shock waves and video double images

In retrospect, I should have noticed this before. With each of Mickey's substructure detonations, shortly after detonation, the video frames display double images for a short time. I have felt the underwater shockwave rock the barge, but never thought much about it until I was reviewing the individual frames. Sparky caught the ignition and intial shock with perfect lighting and the double images became quite obvious today.

To confirm that this was consistent with underwater blasts but not with dropping the above water piers, I looked back at the earlier P-10 substructure blast and the C-4 substructure blast and sure enough, each showed double images 400 - 500 msec after my estimate of the ignition time. For comparison I looked at the C-2 pier blast and no underwater shock wave and no double images.

Video typically consists of a sequence of frames (usually 1 frame every 33 msec) consisting of two fields acquired every 16.6 msec. Each field is composed of a sequence of video lines as the video detector is horizontally scanned from top to bottom. One field consists of the even lines and one field consists of the odd lines.

So if there is movement between the two fields, they will register slighly different images. When combined to form a single frame (with all the scan lines), the image appears as a double image as seen below - at least that is what I think is going on. I would welcome other ideas about what is happening.

Today Mickey and Ken downed the C-1 base with Sparky on the barge with the array of cameras. Here is the story. The worksite - with P-8 in the background - C-2 base to the right and to the right of Arthur's west leg is the C-1 base

Another view of P-9 to the left of Arthur's east leg, P-8 and C-2

Mickey and our camera array

Frame by frame ignition and detonation

T = 0.000 sec        T = 0.033 sec

Note the faint line between C-1 and the right side of the rock island - shock tube excitation
T = 0.067 sec        T = 0.100 sec

T = 0.133 sec        T = 0.167 sec

T = 0.400 sec        T = 0.433 sec

T = 0.467 sec        T = 0.500 sec

T = 0.533 sec        T = 0.567 sec

T = 0.600 sec        T = 0.633 sec

T = 0.667 sec        T = 0.700 sec

T = 0.733 sec        T = 0.767 sec

T = 0.800 sec        T = 0.833 sec

T = 0.867 sec        T = 0.900 sec

T = 0.933 sec        T = 0.967 sec

T = 1.000 sec        T = 1.033 sec

T = 1.067 sec        T = 1.100 sec

T = 1.133 sec        T = 1.167 sec

T = 1.200 sec        T = 1.233 sec

T = 1.267 sec        T = 1.300 sec

T = 1.333 sec        T = 1.367 sec

2:51:48 Before

2:51:49 Ignition

and a closer look at the festering surface

2:51:50 Boom










Going home - almost unbuilt P-9 and Arthur's east leg and to the right P-8 and C-2 and Arthur's west leg

Michael's 1250 in the praying position

Ken's jeans, Mickey's tanned head and his water taxi, heading home

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Attribution: C. Frank Starmer and Sparky Witte from